The actual difference in applied effective power may well be in the bullet construction rather than in velocity or muzzle energy, however derived. Since it long settled that a certain amount of penetration is necessary, the bullet type and construction may well determine penetration at a given velocity.
My solution is to have 2 extra 9mm cylinders fitted to 2 different M60s, one a 3 inch HB, the other a 60-4 with adjustable sights. Seems to work OK so far.